1. Technical Field of the Invention
The present invention relates generally to nondestructive measurement of strength of heat-treated precipitation-strengthened alloys and specifically to the monitoring of the harmonic content of an acoustic signal passed through the specimen during the heat treatment process.
2. Discussion of the Related Art
Generally, heat-treatment is performed according to compiled data. A recipe is followed, according to previous experience, to arrive at a heat treatment time which will produce a maximum strength for a given alloy. The compiled data method assumes, however, that the material being treated is identical to those used to compile the data. This is not usually the case as the material is generally not homogeneous in constituent composition and the composition from batch to batch is generally different. Thus such methods can only provide an estimate as to appropriate heat treatment parameters.
Methods of determining strength are known which are destructive, such as tensile or torsional strength tests. Other methods are not as destructive, but only assess surface strength of the material such as surface hardness tests. These methods are static and generally require that the material be removed from the heat treatment process.
A large class of alloys are strengthened by precipitates which contain a different local chemical composition from that of the bulk metal matrix. It has been well established that optimal strength levels may be achieved in certain alloy systems when relatively large strains become set up at or near the interfaces between the precipitates and the surrounding matrix. The strain fields are very effective in blocking the motion of point and line defects through the metal when a load is applied. From a processing standpoint, the maximum strength is achieved by heat treating alloys at the proper temperature for an optimal length of time. During initial hardening, precipitates begin to cluster together in very small groups known as zones. With increased time, the zones grow larger and are known as particles or precipitates and the strain fields become stronger up to a maximum. Aging for too long leads to a decrease in material strength as the strain fields diminish in strength due to continuing growth of the precipitates.